Electric motor control circuit



Sept. 2, 1947. R. E. PETERSON 2,426,640

ELECTRIC MOTOR CONTROL CIRCUIT Filed March 29, 1943 4 Sheets-Sheet l I i I I W l jig "1I l ELL Fla!

INVENTOR. ROBERT E. PETERSON A TTORNEY DOWN LIMIT Sept. 2, 1947.

R. E. PETERSON ELECTRIC MOTOR CONTROL CIRCUIT Filed March 29, 1943 4 Sheets-Sheet 2 HQIST CONTROL LOWEQ C O NTROL A FIG. 5 3/1 I POWER LINE RELAY CONT C I RCUIT HOIST LOWER RELN RELAY UP LIMIT SWITCH UP DOWN DRIVE MECHANISM 5WITCH INVEN TOR. IQQBERT EPETERSON BYWWW Sept- 2, 1947- R. E. PETERSON ELECTRIC IOTOR CONTROL CIRCUIT Filed larch 29, 1943 4 Sheets-Shoot 3 INVENTOR. ROBERT E. PErERsoN A TTOZ/VEY Sept. 2, 1947. R. E. PETERSON 2,426,540

ELECTRIC MOTOR CONTROL CIRCUIT Filed larch 29, 1945 4 Sheets-Sheet 4 MAIN POWER C mcurr B RAiE L RESEZZING Pfi/ER ol- CONTROL MOTOR CIB'CUIT UNIT CONTROLLER T T Y T Q REMOTE WARNING LAMP C IN VEN TOR. [Zoasm EPmRsoN A TTOBNE Y Patented Sept. 2, 1947 ELECTRIC MOTOR CONTROL CIRCUIT Robert E. Peterson, Danvers, Mass., assignor, by

mesne assignments, to Submarine Signal Company, Boston, Mass., a corporation of Delaware Application March 29, 1943, Serial No. 481,058

12 Claims.

The present invention relates to the control of electrical equipment at points of limit of normal travel, as, for instance, in hoisting and lowering equipment, reciprocating apparatus or apparatus travelling in the defined course.

The invention is particularly useful in connection with hoisting and lowering equipment used in connection with the operation of subaqueous sound equipment for sound ranging and for depth sounding although the mechanism may also be applied to apparatus similarly operated for other purposes.

In submarine sound ranging apparatus it is customary to raise and lower a sound projector through a so-called sea chest in a vessel. When it is desired to use the equipment, it is lowered through the sea chest into the ocean water to the point of its farthest travel where it is seated in proper supporting housing sufficiently strong and steady so that it may be easil rotated on its shaft about a vertical axle. For this purpose limit switches have been employed permitting the hoisting and lowering of the shaft to the desired limit of travel. These limit switches are usually operated through electrical relays which at times stick, failing to shut off the motor driving the hoisting or lowering apparatus and thereby causing considerable damage to the housing structure.

The present invention has for its purpose the elimination of this danger and also the accurate seating of the apparatus in such a manner that danger to the equipment is avoided in spite of variations of hips speed during the hoisting or lowering period.

Further merits and advantages of the present invention will be more fully and completely understood and explained in connection with the drawings showing an embodiment of the invention in which Fig. 1 shows a simplified schematic diagram of a circuit for the invention;

Fig. 2 shows a modification of the arrangement shown in Fig. 1;

Fig. 3 shows a complete schematic diagram of the arrangement shown in Fig. 1;

Fig. 4 showsa schematic wiring diagram of a modification of the diagram shown in Fig. 3; and

Fig. 5 shows the equivalent of the diagram shown in Fig. 4 with the apparatus more completely illustrated in Fig. 4.

In the arrangement indicated in Fig. 1 only the diagram of the control apparatus is illustrated. l and 2 represent the supply-line terminals. 3 represents the upper hoist-limit switch at the end of the upper travel and 4 represents the lower limit switch at the lower end of the travel. It will be understood that while these positions are described as hoist and lowering, any other relationship such as forward and back, right and left or up and down may be used to designate opposite ends of the limit of travel of the moving apparatus. These switches are normally closed in the position indicated in Fig. 1 with the contact bar 5 across the contact points 6 and 6 and the contact bar 1 across the contact points 8 and 8. When the travelling gear is at the upper ends of its limit of motion. the contact bar 5 raised to a position across the contacts 9 and 9' while when the apparatus has travelled to the lower limit of its motion, the contact bar I is across the bars l0 and Ill. The hoist-control switch is represented as H and the lower control switch is represented as L.

H represents the hoist coil and Li a normally closed switch in the hoist line. This switch is mechanically operated with the lower switches lettered L and remains closed through the hoist line while the other L switches remainopen.

The hoist circuit is formed as a series circuit between the supply lines L1 and L2 through the hoist coil H, the switch L1, the hoist switch H and the contacts 6 and 6 which are bridged by the bar 5. When the hoist switch H is operated manually, the hoist coil H is energized which immediately closes the hoist hold switch H2 permanently completing and holding the hoist-closing relay coil H. This relay controls the supply of current to the motor for driving the hoisting mechanism none of which is illustrated'in Fig. l.

A similar arrangement of switches is operable for lowering the mechanism. This comprises the lowering manually operated switch L, the relay coil L, the holding lowering switch L2, the relay coil L, the holding lowering switch L2, the normally closed switch contacts H1 which is closed when the hoist coil H is not energized and the contact bar 1 which bridges the contacts 3 and 8. When the coil L is energized after the switch L is closed, the switch L2 closes and keeps the coil L energized during the lowering operation.

In addition to the switches which have been mentioned there are provided two additional switches La and H3. The L3 switch is mechanically operated in the same manner as the L2 switch and the H3 switch as the H2 switch. The circuit for each of these switches, however, is completed through the limit switch contacts; L3 across the contacts I0 and i0, and H3 across the As a result, therefore, if the contacts 9 and 9.

contacts on lowerin should stay closed due to the sticking of the relay even though the coil L becomes deenergized after the contact bar I is moved away from the contact 8 and 8 by the lowering of the apparatus, a circuit will be closed by the same bar across the contacts l and ID. This will complete a circuit from the terminal 2 through the hoist coil H, the contact L: which is closed due to the assumed sticking, the bar I across the contact points In and Ill to the other terminal I, thus energizing the coil H which closes the power contacts to reverse the rotation of the motor. This energization of the hoist coil 1-1 will react positively to open the L switches La and La and close the switch L1 by a mechanical interconnection between the two relays to be explained later, see Fig. 3.

Since the energization of the hoist coil 1-! operates to close the hoist switch Hz. a circuit will now be completed through the regular hoist control line similarly as when manual operation of the switch H had taken place. The switches are provided with sufiicient delays so that there will be positive closing through the hoist controls as the lowering controls are released. As a result of this arrangement the driving motor instead of continuing to drive the lowering mechanism in the same direction will reverse and send the unit in the opposite direction which will continue in its motion until it has reached its upper limit.

While the operation above has been described in connection with the lower limit switch, the action i similar in connection with the upper limit switch as will be obvious from a consideration of the diagram of Fig. 1.

In the diagram of Fig. 2 the basic circuit is the same as that of Fig. 1 and therefore the basic elements are numbered the same. The relay switch coils are H and L. The lowering and raising switches are L and H, respectively, and the holding switch contacts are L: and H2, respectively. L1 and Hi are respectively the normally closed switch contacts in the hoisting and lowering circuit and operate with L' and H respectively to open when the correspondin L and H switche are respectively closed. The switch L: in Fig. 2 operates in the same sense as the switch L2, and the switch H: in the same sense as the switch H2 as is the case in Fig, 1. In Fig. 2, however, their position in the circuit is slightly difi'erent because of the different arrangement oi the rest of the circuit, as will be explained.

The circuit of Fig. 2 provides a relay 20 with two armature contact blades 2| and 22 which are slow acting and may be damped as indicated by the dash pots 23 and 24. The coil 20 is connected at one end to the line 2 and at the other end to a common junction of the contact switches 25 and 26. The contact switch 28 remains normally closed when the L group of switches is open (L2). The contact 25 remains closed when the H (H2) group of switches is open. When either the L group or the H group operates, then along with this either the switch 22 or 25 operates in the opposite sense. The same applies to the switches L1 and Hi as mentioned above. Single contact bars 21 and 28 bridge the contacts 29, 29' for the lower limit switch and 30 and 30' for the upper limit switch; the mechanism reaching it limits opens the circuit by pushing one or other of the bars 21 or 28 away from the contacts which they bridge. Normally the coil 20 is energized by the circuit completed from the terminal through the coil 20 through 4 either of the closed contacts 25 or 26 and across one of the bars 21 or 28 to the terminal 1. Under these conditions the armature contacts 2| and 22 are held away from the fixed contact points 21 and 22. Under these conditions neither the H coil nor the L coil need be energized. If it is assumed, however, that the mechanism is at the top or its travel, then the bar 28 will not bridge the circuit between the contacts Ill and 20'. Ii the lowering switch L i pressed under these conditions, then the main circuit will be completed through the L coil and the switch bar 2!. This will close the contacts La. La and open the contacts L1 and 28, all'oi' which mechanically-operate together. The-coil 20 will therefore remain energized but only through the closed switch 22 and the contact her 21 bridging the contacts 22 and 2!. The coil H will be unenergized because the switch L1 is open and because the armature bar 22 is away from the contact point 22.

As in the operation of Fig. i, the coil Ll closes the motor circuit and sends the mechanism downward until it reacts against the limit switch and pushes the bar 21 away from the contacts 28 and 29. This action will deenergize the main circuit, deenergizing the coil L and if the L lWltChES do not stick, La and La will open and L1 and 28 will close and the coil 20 will remain energized, thus ceasing the operation of the entire mechanism.

If the relay controlled by L sticks and the ontacts Ln and 10 remain closed, then the following action will take place: The relay coil 20 will become deenergized since the switch 22 will remain open, and while the switch 25 remains closed, the circuit has been broken through the limit switch when the contact bar 21 leaves the contacts 28 and 29'. The armatures 2| and 22 will, after a short time interval, make contact with the contact points 2! and 22. This will complete the circuit through the coil H in the loilowing manner: From the terminal 2 through the coil H, through the contact points 22, the armsture 22, the switch In which is closed because of the assumed sticking oi the relay, and the contact bar 22 bridging the contacts 2| and 22' in the other line terminal I. The coil L will remain deenergized at the same time. The closing of the circuit between the elements 22 and 22 will produce a positive action on the relay H and due to mechanical interconnection between the two relays H and L, will act to open the contacts In and La and close the contacts 26 and L1 at the same time through the action of the coil H. The switches H: and H: are then closed. and 22 and H1 opened, thus completing the holding circuit through the coil H in the following manher; from the terminal 2 through the coil H, the switch L1, the switch Hz, the contact bar 22 across the contact terminals 22 and 22' and the line I. The coil L will remain deenergized since the switch H1 is opened and also the contact between the elements 2| and II since the coil 2015 now energized through a circuit comprising the terminal 2, the coil 20. the switch 22 and the contact bar 28 across the contacts I. and II to the terminal I.

In the same manner as previously described in connection with Fig. 1, the mechanism will, after opening the contacts at the bar 21, reverse its direction of travel and travel towards the top limit switch.

The arrangement described in connection with Figs. 1 and 2 is shown with schematic operating elements in Fig. 3. In Fig. 3 the manual opersiting-lowering switch is indicated by L corresponding to the same letter in Figs. 1 and 2, and the raisin switch by H corresponding also to the same letter in Figs. 1 and 2. The powersupply lines are shown at 33 and these run both to the switch 34 for lowering and the switch 33 for raising, as indicated by the three contacts 33. 33, 33 on the switch 34 and 31, 31, 31 on the switch 35 from which the power is conducted over the line 33 to the motor 33. Each switch closes the circuit in phase relationship such that the operation is down when the lowering switch is closed and up when the raising switch is closed. The lowering relay coil L is schematically shown as operating an armature 33 to pull down the arm 33 carrying the upper elements of the switch 33 for closing the motor circuit and the upper elements of the switches In and L as well as one of the contacts of the switch L1 shown closed in the figure. The action of pulling down the armature 33 at the right in Fig. 3 raises the armature at the left pushing upwards the supporting arm 43" which carries the corresponding switch contacts for the hoisting switch. A pair of balancing springs 4| and 42 may be used to balance the operation of the switch arms one against the other. The raising switch 35 has a set of contacts H: and H: which are shown open and H1 which is shown closed. The wiring circuit for these is exactly as shown in Fig. 1.

It may be noted in Fig. 3 that the ends of the armature 33 rest in recesses 53 and BI of the supports 43 and' 40', respectively, and that at the edge of these recesses there are provided small plates 52 and 53 carrying adjustment screws 34 and 33, respectively, for adjusting the operation of the switch. The switch may be adjusted in operation so that as one relay coil operates to draw down its end of the armature 33, the contacts on the otherswitch arm plates which are closed will open before the new group of contacts are made.

Under these conditions instead of the motor reversing and the gear being driven in the opposite direction, the coils H and U will both become deenergized and the apparatus come to rest at the limit of operation.

The up-down drive motor 33 is illustrated as operating through a worm 43, a gear 44 carrying a sprocket wheel 45 which drives the chain 43. The mechanism 41, which is lowered, is mounted on a shaft 43 which is driven up and down to the plate or bar 43 which has one end fixed to the driving chain 43. The plate 43. as illustrated, carries a contact finger which contacts either the bar 3 in the upper position or the bar 1 in the lower position. These bars are normally tensioned by springs 3' and 1' against the first set of contacts 3, 3' and 3, 3, respectively. The operation of the mechanism has already been explained in connection with the description of the circuit in Fig. 1. Simultaneously with the closing ofthe contact 33, the contacts La and La close while the contact Ll opens. This same action positively insures opening of the switch contacts 31, H2 and H: and the closing of Hi. This, then, follows in operation the exact arrangement as described in connection with Fig. 1.

The same arrangement as shown in Fig. 3 may also be applied to the circuit as illustrated in Fig. 2.

In Figs. 4 and 5 an arrangement is shown in which a brake-operating mechanism is used in connection with the limit switch circuits as p eviously discussed. In Fig. 4 the same lettered and numbered characters refer to the same lettered and numbered elements as in Figs. 1 and 2. The motor in Fig. 4 for the up and down drive 33 is the equivalent of the motor shown in Fig. 3 except that it has a magnetically operated brake mechanism 33 which is controlled through a brake elay 3| having a group of contacts 32 for operating the brake coil and a contact 33 for controlling the protective relay coil 34 the function of which will be described later.

Fig. 4 also shows the reversing motor rela 35 which, as indicated in Fig. 3, is operative through the hoist and lower relay coils H and L. A stop switch 33 is also incorporated in Fig. 4. The limit switches in Fig. 4, 31 for the down limit switch 33 for the upper limit switch, are the same as the limit switch 33 shown in Fig. 5. As indicated in Fig. 4, when the mechanism is traveling against the down switch, first one group of contacts 10 is opened by the bar 1| and then a second group of contacts 12 by the bar 13. A similar arrangement is used in the hoist limit switch. The bars 14 and 15 open the circuit between the contacts 13 and 11, respectively. In the sequence followed in the operation of Fig. 4, first the motor becomes disconnected and is allowed to coast freely, then the brake is applied as the unit seats itself at the ,end of its motion. The operation of the circuit will be understood by a consideration of the operation of elements as the mechanism goes through its normal travel.

The adjustment of the switches is such that the brake is applied at such a time as to absorb considerable of the stored energy in the coasting motor armature when the ship is not moving. Thus, when the ship is underway with high speed,

the energy is sufllcient to carry the mechanism.

all the way down to the fully lowered position against the side thrust on the mechanism due to motion through the water. Thus, the brake acts as a regulation to insure completed lowering operation despite speed differences.

Assume that is desired to send the mechanism to the lower end of its course either when it is at the top or at some intermediate place. The lowering button Lis pressed and power is thereby supplied across the line I, at the left of Fi 4 through the relay coil L in the following manner: over the line 13, 13 and 33 to the L coil through the L1 coil, the lower switch L, across the bar II to the line 3| through the stop switch 33 and the line 32 to the terminal I. The energization of the coil L closes the switches L2 and La, opening the switch L1. The switches H: and H3 are assumed already to be open and the switch Hi to be closed. Simultaneously with the energizing of the coil L a circuit is completed across the coil n which is connected in shunt across the coil L by means of the lines 33 and 34 connected to one end of the coil and the line 33 from the other end of the coil to a terminal of the coil L'. This action closes a circuit across the relay contacts 33 which is bridged by the armature 31 so that the brake coil 3| becomes energized through the armature bar 31, the line 33 and 33 going to one end of the relay coil and. the line 33 going from the other end of the relay coil to the line 3| which connects through the stop switch 33 to the line 32 and the terminal This action closes the relay contact 32 of the brake control unit retracting the brake Plate 32, Fig. 5, against the action of the spring permittin the up-down motor 33' to drive the unit freely in the desired direction. The operation of the brake-control unit also closes the contacts 33 which energizes the coil 64 connected by means of the lines 18 and 9| across the terminals I and 2. The energizing 01 the coil 64 swings relay armature 93 clockwise as viewed in Fig. 4 breaking the circuit across the contact points 94, thus assuring that the line 95 is not connected to the supply terminal I. As the mechanism travels to its lower limit, it first opens the circuit across the contact III. This will deenergize the coils L and n permitting the armature 81 to bridge across the contacts 86'. Since the coil 64, however, is still energized at this point, there is no connection across the contact points 94. As the down mechanism progresses further and opens the circuit across the contacts I2 by pushing away the bar I3, the circuit to the brake-relay coil GI is broken. This circuit is traced in the following manner; from the terminal 2, the line 96, contacts II, the bar I5, line 98, contact 12, bar I3, line 99, line 88, line 89, relay cOil 6|, line 90. line 9|, stop switch 66, line 82 to terminal I. The circuit is broken at the contact 12 and the brake coil 69 is thereby released permitting the springs to let the brake plate 92 act in stopping the motor. If, however, at this point the down relay has stuck so that the contacts L2 and L: are still closed. the circuit to the hoist relay will be closed in the following manner from the terminal 2, the lines I9, I9 and 89, through the coilli, the contact L: which is now presumed to be closed, the contacts 86 and the bar 81', the line 95, the contacts 94 which are now bridged by the armature 93, the line 9|, the stop switch 66, the line 92 to the terminal I.

The energization of the coil H will simultaneously energize the coil U and operate to close the switches H2 and H: while the switch Hi will be open. The energizing of the coil U will draw the armature I and IIII over to the right and close a circuit across the contacts I02 so that the coil 6| of the brake relay will again be closed, thus permitting the motor to act freely in driving in the opposite direction or the direction of raising.

The switch at the upper end will operate in a similar manner in event that the H relay sticks after both the bars I4 and 15 have opened the limit switch circuit. In this case the lowering coil will be energized to reverse the motor and again the brake-relay coil will be energized so as to relieve the motor of the friction of the brake.

In the drawing of Fig. 5, the elements of Fig. 4 are somewhat schematically illustrated. The motor 39 driving a Worm shaft III) and worm wheel III raises and lowers the projecting gear II9. A long screw II2 connected with the hoisting and lowering gear drives a worm Il3 which meshes with the worm gear I I4 on which the limit switch 69 has one element mounted. The switch 5 corresponds to the contact bar II and the contact of Fig. 4, and the switch IIB corresponds to the contact bar 13 and contact 12 of Fig. 4, while the switch I I1 corresponds to the contact bar 14 and contact 16 and the switch II8 to the contact bar and contact I1. The box labelled "Reversing motor controller includes the relay controls for the H and L switches of Fig, 4, while the brake-control unit includes the relay-brake coils and switches and also the relays n, U and 84 with their contacts,

Having now described my invention, I claim:

1, A limit-switching system for a means adapted to be propelled towards a limit established by the position of the limit switch comprising a holding relay having contacts adapted to complete the circuit for propelling the unit towards the limit,

means operative upon reaching the limit for breaking the circuit to said holding coil and normally deenergizing the propelling means, and means operative by further advance of the propelling means if said propelling means is not deenergized for reversing the drive of said propelling means.

2. A limit-switching system for a means adapted to be propelled in a given line 01' travel between two limiting positions at the end of the travel comprising independently manually operated switch-circuit means for driving said propelling means in one direction or the reverse thereof, said means including relay-holding means adapted to hold said circuits closed for driving in the desired directions, means Operative upon reaching the limit or drive in either direction for breaking the circuit to said holding coil and normally deenergizlng the propelling means, and means operative by further advance of the propelling means if said propelling means is not deenergized for reversing the driving of the propelling means.

3. A limit-switching system for a means adapted to be propelled in a given line of travel between two limiting positions at the end of the travel comprising independently manually operated switch-circuit means for driving said propelling means in one direction or the reverse thereof, said means including relay-holding means adapted to hold said circuits closed for driving in the desired directions, means operative upon reaching the limit of drive in either direction for breaking the circuit to said holding coil and normally deenergizing the propelling means and switching means, cross connecting said holding relay means for driving the firstmentioned means in a direction opposite to that in which it is being propelled, and means operative at the limit of the drive for operating said switching means for reversing said drive.

4. A limit-switching system for raising and lowering sound gear into and out of a vessel having limit switches for controlling the seating of the apparatus in its end position comprises circuit holding means operative to control the operating of the driving means in either direction, and means controlled by said limit switch if the limit switch fails to function for changing the control of said circuit holding means for reversing the drive of said driving means.

5. In a limit-switching system for a. means adapted to be propelled in a given line of travel between two limiting positions, a limit switch adapted to be opened bpffihe means upon reaching its limiting position, holding relay means and switches operated thereby, said means having a reversing switch and means operative by the propelled means pressing forward of its limiting position [or causing the relay means to operate said reversing switch.

6. In a limit-switching system for a means adapted to be propelled in a given line of travel between two limiting positions, a limit switch adapted to be opened by the means being propelled upon reaching its limiting position, holding relay means and switches operated thereby, said means comprising duplicate switching means operatively associated with each other whereby the operation of one set in one sense operates the other set in the other sense and means contained in the limit switch operative if the first-named means remains energized at the limit of its travel for causing the operation of said relay means in its reverse sense to reverse the energization of said first-named means.

'1. In a limit-switching system for a means adapted to be propelled in a given line of travel between two limiting positions, a limit switch adapted to be opened by the means being propelled upon reaching its limiting position, holding relay means and switch operated thereby, said means comprising duplicate switching e1ements, one group of each of said duplicate elements adapted to reverse the direction of propulsion of said first-named means and the other group of said switching elements adapted to reverse the operation of said holding means if the propelled means continues to be operative at the limit of its travel whereby the drive of said propelled means is reversed.

8. A limit-switchlng system for raising and lowering sound gears into and out of a vessel having limit witches for controlling the seating of the apparatus in its end positions comprising a pair of limit switches operatively associated with the sound gear reaching its limiting positions,

. a motor for propelling the sound gear in its raising and lowering, a brake means, means operative when the sound gear reaches its limiting position for first deenergizing said motor and secondly for applying a brake thereto as the sound gear reaches its limiting positions.

9. A limit-switching system for raising and lowering the sound gear comprising a driving motor, a brake device, an operating circuit and a plurality of limit switches operatively associated with the approach of the sound gear to its limiting position, said operating circuit including means associated with the limit switches for deenergizing said motor upon reaching its limiting position and means operative with the deenergizing of said motor by the position of the gear in relation to the limit switch for applying the brake to said motor.

10. In a hoisting and lowering apparatus having sound gear at the bottom thereof including a limit-switching system for raising and lowering the sound gear, a driving motor, a brake device, an operating circuit and a plurality of limit switches operatively associated with the approach of the sound' gear to its limiting position, said operating circuit including-means associated with the limit switches for deenergizing said motor upon reaching its limiting position and means operative with the deenergizing of said motor by the position of the gear in relation to the limit switch for applying the brake to said motor and holding relay means included in said operating circuit having elements for reversing the direction of the operation of the motor if said driving motor fails to be deenergized.

11. In a hoisting and lowering apparatus having sound gear at the bottom thereof including a limit-switching system for raising and lowering the sound gears into and out .of a vessel, limit switches for controlling the seating of the apparatus in its end positions comprising a pair of limit switches operatively associated with the sound gear reaching its limiting positions, a motor for propelling the sound gear in its raising and lowering, a pair of relays having associated groups of switches for reversing the operation of the propelling motor and means operative through said relays for opening one group before the other group is closed when the sound gear has reached its limiting position whereby the gear is brought to rest in its limiting position.

12. A limit-switching system for reciprocating a travelling element between limiting positions including a limit switch adapted to be operated when'the travelling element has reached its limiting position, motor meansadapted to drive said traveling element in either direction, relay and switch means operatively associated with said limit switch for selectively driving said travelling element in one or the other direction, and circuit means connected therewith energized through the operation of said limit switches for operating said relay means to reverse the direction of drive of said motor.

ROBERT E. PETERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 650,973 Hill June 5, 1900 1,215,573 Otis Feb. 13, 1917 1,768,650 Wood July 1, 1930 1,839,934 Stansbury Jan. 5, i932 

